The turbines and compressors of gas turbine engines such as jet engines each include one or more circumferentially extending rows or stages of rotating rotor blades which are axially spaced between rows or stages of fixed stator vanes. Each rotor blade has a blade root mounted to the rotor disk, and an air foil extending radially outwardly from the root which terminates at a blade tip. In many gas turbine engine designs, a number of abutting, circumferentially extending shroud segments are carried by the turbine or compressor case to form an essentially continuous cylindrical-shaped surface along which the tips of the rotor blades tangentially pass. Each of these shroud segments includes an outer face, and an inner, arcuate-shaped face along which the blade tips pass, opposite end portions which abut with adjacent shrouds and opposed side mounting rails which mount to stationary hangers on the casing of the turbine and/or compressors.
The shroud segments, particularly those located in the turbine of a jet engine, are subjected to high temperatures at their inner face along which the rotor blades pass. In an effort to lower the temperature of the shroud segments and increase their durability, cooling air from an intermediate stage of the compressor is often directed onto the outer face of the shrouds. This cooling air is intended to reduce the overall temperature of the entire shroud without directly contacting the inner face and disrupting the air flow through the turbine or compressor.
A major design consideration in any jet engine is the reduction of specific fuel consumption. One source of decreased specific fuel consumption in many jet engine designs is pressure losses resulting from the creation of a relatively large radial tip clearance between the tip of the rotor blades and the inner face of the shroud segments. It is believed that one source of increased radial tip clearances is a problem known as "chording". Chording results from the temperature differential between the high temperature inner face and the cooler outer face of the shroud segments. Impingement of cooling air on the outer face of the shroud segments while the inner face is subjected to high temperatures causes the shrouds to chord or "straighten out" circumferentially, i.e., the end portions of the inner face of the shroud tend to move radially outwardly relative to the center portion of the inner face of the shroud. While the interconnection of the side mounting rails of the shroud segments with the stationary hooks on the case of the compressor or turbine is intended to resist chording or "straightening-out" of the shroud segments, such resistance is overcome by the temperature gradient between the outer and inner faces thereof.
In effect, each individual shroud segment behaves as a curved beam and tends to straighten-out circumferentially from end-to-end in response to the radial temperature gradient. As a result, a wedge-shaped space or gap is created between the tip of the rotor blades and each end portion of the inner face of the shroud segments as the rotor blades are moved therepast. Such chording can also cause additional blade tip rubs in the central portion of the shroud segment inner surface. These rubs produce friction which further increases the radial temperature gradient, thereby causing even further chording and rubs. This increase in radial tip clearance at the end portions of each shroud segment has been found to be equivalent to a uniform tip clearance increase of about 0.004 inches in some types of jet engines, resulting in the significant reduction of specific fuel consumption, e.g., of about 0.4%.
One attempt to reduce chording, or the straightening out of the shroud segments, has been to form one or more radially extending notches or grooves in each of the side mounting rails of the shroud segments which mount to stationary structure of the turbine or compressor casing. These radial grooves are intended to reduce or eliminate the "beam strength" of the shroud segments by making them discontinuous along the length of their side mounting rails.
It has been found that the presence of radial notches or grooves in the shroud segments creates high stress concentrations at the inner end of such grooves. These stress concentrations can create cracking or fracturing of the shroud segments which can propagate from the groove and result in premature failure of the shroud segment.